The present invention relates to internal combustion engines, such as used in vehicles, and more particularly direct injected compression-ignition (CI) engines using clean low viscosity fuels such as dimethyl ether (DME) to achieve ultra-low emissions of oxides of nitrogen and particulate matter.
The background of air pollution problems caused especially by internal combustion engines in trucks and buses as well as proposed solutions to such problems are set forth in detail in my U.S. Pat. No. 5,816,228, entitled FUEL INJECTION SYSTEM FOR CLEAN LOW VISCOSITY FUELS, the disclosure of which is incorporated herein by reference. One promising solution involves the use of DME as an alternative fuel for CI engines. However, conventional fuel systems are not suitable for use with DME because of the low viscosity of such a fuel. As disclosed in U.S. Pat. No. 5,816,228, a pressurized fuel storage tank is required, as well as a special pressurized fuel injection system.
A major problem with using DME is internal leakage of DME within the fuel injection system of a CI engine. In the fuel injection system as disclosed in U.S. Pat No. 5,816,228, DME is supplied under pressure to an accumulator, also called a common rail, and then is fed through a solenoid operated control valve to a fuel injector. When the engine is shut down, fuel can leak from the accumulator to the fuel injectors since the control valves are of the spool type. The fuel injector nozzle valves are precision steel valves, but they form imperfect seals for DME. Accordingly, when the engine is shut down, fuel can also pass through the fuel injector nozzle valves over a period of time and enter the associated combustion cylinders of the engine which is not acceptable. Accordingly, leakage from the accumulator to the fuel injectors must be prevented.
Even if means is provided for effectively preventing DME from leaking from the accumulator into the fuel injectors after the engine is shut down, a small volume of fuel will be trapped in the injectors upon engine shut down which could eventually leak into the combustion chambers of the engine. This type of leakage must also be prevented.